Thin film resistors are utilized in electronic circuits in many important technological applications. The resistors may be part of an individual device, or may be part of a complex hybrid circuit or integrated circuit. Some specific examples of thin film resistors in integrated circuits are the resistive ladder network in an analog-to-digital converter, and current limiting and load resistors in emitter follower amplifiers. Film resistors can comprise a variety of materials including tantalum nitride (TaN), silicon chromium (SiCr), and nickel chromium (NiCr). These resistor materials are chosen for their resistivity properties and stability, especially with respect to the property of temperature co-efficient of resistance (TCR). Typical values of sheet resistivity and TCR for 10 nm films of thin film resistors are 2000 ohms per square (&lt;200 ppm/.degree. C. TCR) for SiCr and 200 ohms per square (&lt;100 ppm/.degree. C. TCR) for NiCr.
Laser trimmable thin film resistors are well established in the prior art and are an important enabling technology for devices such as analog-to-digital converters with highly monotonic outputs. Laser trimming is necessary in many present day integrated circuits, especially as device tolerance specifications increase. Untrimmed wafers either do not have a high enough yield to allow production of a low cost part or the accuracy of the resistors or other components required in the device is too high to be achieved by process tolerances. The most common approach used today in trimming resistors incorporated in integrated circuits uses functional trimming whereby the circuit is rendered functionally operable, usually at the wafer level using an appropriate probe card. Device parameters such as voltage level are monitored constantly as a laser cuts or removes certain portions of the thin film resistor according to a pre-determined trim algorithm. Once a predetermined set of outputs is reached, trimming is stopped.
As well as the above mentioned desired properties of SiCr and NiCr, these materials are also suitable for trimming in completely closed environments, that is, even after they have passivation layers deposited over them and the final steps in processing the wafer containing the devices and resistors have been completed. In this type of laser trimming, the resistor material is not completely physically vaporized, but rather the heat induced by the high intensity laser either partially changes the morphology of the heated part and thus grossly changes its conductivity properties or causes the elements in the resistor to undergo chemical reactions such as oxidation thus also grossly changing the conductivity of the material and creating a highly insulating region. It is important that during this step the laser beam intensity or pulse fluence is uniformly absorbed into the resistor layer at all areas across the wafer. If the absorption into the resistor varies considerably, some resistors in low absorption areas may not be trimmed effectively due to incomplete chemical reaction or morphological change. Other resistors in high absorption areas may absorb too strongly and cause vaporization of the resistor material causing it to remove the overlying passivation layer.
It is well known that the most important factor governing the absorption of the laser into the resistor is the nature of the multilayer dielectric stack in which the resistor is formed. With reference to FIG. 1, a thin film resistor 10 is contained between two stacks of dielectric material 20 and 30. This stack is deposited on a silicon wafer substrate 40. Since the thickness of the dielectric stack films are on the order of the wavelength of the incident laser beam which is highly monochromatic, transmitted and reflected laser beams within the stack can either have a constructive or destructive interference in the area around thin film resistor 10. FIG. 2 shows the reflectivity spectrum for a stack similar to the stack shown in FIG. 1 with the following materials and parameters: a silicon substrate (with refractive index n=3.5, extinction coefficient k=1.7.times.10.sup.-4 ), upon which 10,000 .ANG. of silicon dioxide (n=1.48, k=0) is deposited, followed by 100 .ANG. of NiCr (n=2.63, k=4.28), followed by a final 10,000 .ANG. of silicon dioxide (n=1.48, k=0). According to FIG. 2 the minimum reflectivity points are at 8500 .ANG. and 12000 .ANG. over this wavelength range. These points represent the wavelengths at which the SiCr thin film resistor will absorb most strongly and therefore represent the most suitable laser trimming wavelengths. Additionally, since the derivative of laser absorption is zero at these points, this makes these wavelengths further optimized with respect to repeatability of laser absorption across a wafer. It is noted that any variation in film thickness across the wafer will have the least impact on absorption variations because of the zero differential condition. This is in contrast to points midway between the reflectivity peaks and troughs of FIG. 2, corresponding to the wavelengths where small changes in the thickness of the dielectric layer across the wafer will lead to large and undesirable changes in SiCr absorption.
The thickness of both dielectric stacks 20 and 30 are important in determining the optimum stack parameters. In general, the lower dielectric stack 20 should be an optical quarter wave plate. This means that its physical thickness multiplied by its refractive index should be equal to an odd integer times a quarter of the trimming laser's wavelength. The contribution of the lower layer's importance to the overall cavity increases for thin film resistors that do not absorb the laser substantially. This is almost always the case for resistor materials suitable for trimming like SiCr and NiCr., where the optimum thickness range for trimming has been determined to be between 5 nm and 20 nm.
The upper layer should also be optimized as an anti-reflection coating for the laser wavelength and will similarly be an odd integer of the quarter wavelength. Thus a cavity can be designed to suit the laser wavelength. The most suitable lasers for trimming thin film resistors are presently based on the well known laser activator ion neodymium 3+. Depending on the host lattice into which the neodymium 3+ ion is incorporated or the design of the laser cavity, the main lasing line wavelength can be changed. For example, in the host lattice Y.sub.3 Al.sub.5 O.sub.12 (YAG) the lasing wavelength is at 1064 nm, and in the host lattice YLiF.sub.4 (YLF) it is at 1047 nm.
It is therefore highly desirable to be able to precisely control the thickness of both the dielectric films 20 and 30 in manufactured wafers such that the total dielectric stack containing the thin film resistor gives the beneficial absorption and reflection properties necessary for efficient laser trimming and without using excessive laser powers that may damage circuit components. However, in an integrated circuit manufacturing environment, where dielectric stacks 20 and 30 may be composed of many different layers containing additive thickness errors, it is difficult to precisely control the thickness of these layers to satisfy these requirements.